Urban flood susceptibility mapping in Ilorin, Nigeria, using GIS and multi-criteria decision analysis

The objective of this study is to assess and map flood hazard zones in Ilorin (North-Central, Nigeria). Flood susceptibility zones were identified by utilizing the multi-criteria decision analysis (MCDA) technique integrated with Geographic Information System (GIS). Eight factors considered the most influential variables for runoff accumulation and stagnation in the city during excess rainfall and surface water runoff were taken into account. These factors include elevation, slope, topographic wetness index, convergence index, drainage density, altitude above channel, land use, and rainfall. To obtain the weights of each factor, the Analytical Hierarchical Process (AHP) was applied. The final flood hazard assessment and mapping were done using the Weighted Overlay operation. Essential findings of this study reveal that a critical percentage of the area (38.4%, corresponding to 127.01 km2) has a high flood risk. About 20.5% (67.85 km2) area shows moderate flood risk, while approximately 35.6% (117.77 km2) and 4.2% (13.98 km2) areas are identified as low and very low flood risk areas, respectively. The high flood risk zones need immediate attention because they are mostly found in the densely populated low-lying gentle slope (< 5o) areas. Accuracy assessment of the flood-hazard map indicates a considerable high value (87.5%). A combination of flood susceptibility and vulnerability has shown that about 35% of the metropolitan area is under the threat of high flood risk. The result of this study may be useful for the residents of the city, decision-makers, and urban planners for sustainable flood disaster planning and management in Ilorin.